The Ruling on the Fixed Sunnah Prayers
Question :
What is the ruling on the Sunnah prayers - that is the two Rak'ahs after the prayer?
Answer:
Allah has made it incumbent upon the Muslim to perform five prayers every day, and the Prophet legislated for his people the voluntary prayers before and after the obligatory ones and at all times, except those times when prayer is prohibited.
Included among the aforementioned prayers are the fixed Sunnah prayers and they are: two Rak'ahs before the Zuhr prayer and two Rak'ahs after it; two Rak'ahs before the Fajr prayer; two Rak'ahs after the Maghrib prayer; and two Rak'ahs after the 'Isha' prayer.
These Rak'ahs are the Sunnah and not obligatory - the one who offers them is rewarded but the one who does not offer them is not punished. And they help to accustom a person to performing acts of worship and they are an evidence of love for prayer, and they also make up for any defect or omission in the obligatory prayers. So whoever leaves them sometimes is not guilty of any sin, but to continually leave them is a proof of lack of interest in worship and the one who does so is considered less than just due to his desire to abandon the Sunan and his disdain for righteous deeds.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 2 Pages 296-297